1. Field of the Invention
The present invention relates to a compressed magnetic powder core and, more particularly, to a powder core having a high magnetic flux density and good frequency characteristics of magnetic permeability.
2. Description of the Prior Art
Semiconductor switching elements (e.g., thyristors and transistors), turn-on stress buffer reactors, commutating reactors, energy storage reactors or matching transformers have been used as conventional electrical elements in power transformers (e.g., AC/DC converters, DC/DC converters such as choppers, and AC/AC frequency converters) or in electrical equipment such as noncontact switches.
Such conventional reactors and voltage transformers require an iron core having good magnetic characteristics in a high-frequency range.
Currents having switching frequencies of either several tens of Hz to 200 kHz or several tens of kHz or 500 kHz or more, often flow in conventional reactors and voltage transformers. Therefore, demand has arisen for an iron core which has a low iron loss and whose magnetic permeability is not reduced in a high-frequency range.
An eddy current loss among iron loss components in AC excitation of an iron core increases proportionally to the square of frequency when a magnetic flux density remains the same. Most of the iron loss is accounted for by the eddy current loss in the high-frequency range. As a result, the iron loss is increased and the magnetic permeability is decreased in the high-frequency range.
In a conventional iron core made of a metallic magnetic powder, a decrease in iron loss is achieved by improvement of electrical insulation between the magnetic particles.
Typical conventional iron cores having good high-frequency characteristics are exemplified by so-called dust cores as described in Japanese Patent Nos. 88779 and 112,235.
Although such dust cores have good high-frequency characteristics, their magnetic flux density is low. For example, a maximum magnetic flux density at a magnetizing force of 10000 A/m is only 0.125 T.
In another conventional iron core having a metallic magnetic powder and a binder resin as disclosed in Japanese Patent No. 670,518, good frequency characteristics and a high magnetic flux density can be obtained.
Generally, in the iron core manufactured by compression molding a metallic magnetic powder, magnetostriction caused by compression increases a coercive force as compared with that prior to compression. In addition, a hysteresis loss is increased accordingly.
In order to obtain a low-loss iron core, magnetostriction must be eliminated. For this purpose, a heat treatment (annealing) is normally performed to effectively eliminate such magnetostriction. In the iron core having the binder resin, however, the resin is decomposed or degraded during the heat treatment, and electrical insulation between the metal magnetic particles cannot be guaranteed. It is thus difficult to manufacture an iron core having a low iron loss.